Optimize read-only middleware so that it does not consume as much CPU

Closes #40185 and gitlab-com/infrastructure#3240

See merge request gitlab-org/gitlab-ce!15504

(cherry picked from commit 6c339816)

3c52e2f0 Optimize read-only middleware so that it does not consume as much CPU
cba68d33 use `Gitlab::Routing.url_helpers` instead of `Rails.application.routes.url_helpers`
91075c82 check for `read_only?` first before seeing if request is disallowed
aeb2f49f Revert "check for `read_only?` first before seeing if request is disallowed"